1. Field of the Invention
This invention relates to image reading apparatus, particularly those adapted for reading an image at high speeds.
2. Related Background Art
The structure and operation of a conventional image reading apparatus will now be described with reference to FIGS. 8-10(a).
In FIG. 8, reference numeral 1 denotes a photoelectric transducer which reads image information on an original document 2; 3, a light source which illuminates the original 2; 4, a motor to feed the original; 5, a power transmission belt; and 6, paper feed rollers which feed document 2 line by the rotation of motor 4.
Motor 4 is controlled by a Motor signal output by a motor drive circuit 7 which in turn is controlled by a Start signal output from a CPU (central processing unit) 8. The light from the light source 3 reflected by the original 2 is sensed as image information by transducer 1 via a lens 9. Transducer 1 has a time for which image information is photoelectrically converted and stored as electric charges (hereinafter referred to as the storage time). CPU 8 outputs to the transducer a timing signal SH having a period equal to the storage time to control the storage and outputting of image data in and out of the transducer. Image data output from the transducer synchronously with the timing signal SH is written alternately into buffers A and B synchronously with write signals Ain and Bin output from CPU 8 and read out alternately synchronously with read signals About and Bout.
The timing signal SH is inherently determined by the storage time of the transducer. Alternate writing and reading of the image data sensed by the transducer 1 into and out of buffers A and b synchronously with the Motor signal serves to eliminate a time delay in the data transfer due to the storage time of the transducer.
Although the above image reading apparatus moves the read position via a drive system including motor 4 and force transmission belt 5, the delay in the response time of the drive system is not considered. Therefore, when the time delay caused by the drive system corresponds to, for example, one storage time of the transducer 1, high speed intermittent reading would involve reading unnecessary data which disturbs the output image. FIG. 10(a) shows an image formed by printing out the image data obtained by intermittently reading every 64 lines at high speeds, using this conventional apparatus. The timing chart of FIG. 9 shows the relationship between respective signals and output data where the time delay of the drive system corresponds to one storage time of the transducer. Because the time delay of the motor corresponding to one storage time occurs, the transducer reads image information in accordance with a timing signal SH having the same timing as a Start signal output by CPU 8 to motor drive circuit 17. Therefore, not the data in lines a, b, c to be read, but the data in lines a', a, b, and not the image data in lines d, e, f, g, but the data in lines d', d, e, f, are read and output so that disturbances in the output image as shown in FIG. 10(a) will occur.